docs: def fmt_ces(c, title=None):
      """Format a |CauseEffectStructure|."""
      if not c:
          return '()\n'
  
      if title is None:
          title = 'Cause-effect structure'
  
      concepts = '\n'.join(margin(x) for x in c) + '\n'
      title = '{} ({} concept{})'.format(
          title, len(c), '' if len(c) == 1 else 's')
  
      return header(title, concepts, HEADER_BAR_1, HEADER_BAR_1)